We will isolate and characterize a group of proteins (mol. wt. 56,000 to 60,000) called Tau antigens from papovavirus-transformed cells. These proteins are different from the previously studied viral tumor antigens (T Ag's) in that they are synthesized predominantly in transformed cells rather than in infected cells and appear to have only a limited number of sequences in common with T Ag molecules. They can, however, like T Ag, be immunoprecipitated from extracts of virus-transformed cells using anti-T serum. Furthermore, the SV40-specific Tau antigens share two methionine-labeled tryptic peptides with the large (80,000 daltons) species of SV40 T Ag and at least one with the small (20,000 daltons) species. We propose to investigate the properties, function and biosynthesis of Tau antigens. We will study the composition of Tau antigens and determine if they are associated with other components in the cell. Information regarding the origins of these proteins will be obtained by comparing tryptic peptides of Tau antigens isolated from 1) various strains of SV40-transformed mouse cells, 2) SV40-transformed cells of different species and 3) cells transformed by different papovaviruses. Furthermore, the relationship between T and Tau antigens will be examined in detail to identify and map all homologous sequences. In this regard, we will look into the possible immunologic relatedness of these two proteins. Messenger RNA molecules which code for the SV40 Tau antigens will be analyzed for viral sequences. "Mapping" studies will locate these sequences on the SV40 chromosome. Finally, in order to evaluate the possibility that these proteins are involved in the transformation process, T Ag positive revertant cell lines which have lost the transformed phenotype will be isolated and examined for the presence or absence of Tau antigens.